In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system. These mechanisms include low-level address space management, thread management, inter-process communication. If the hardware provides multiple rings or CPU modes, the microkernel may be the only software executing at the most privileged level, referred to as supervisor or kernel mode. Traditional operating system functions, such as device drivers, protocol stacks and file systems, are removed from the microkernel itself and are instead run in user space. In terms of the source code size, microkernels are smaller than monolithic kernels; the MINIX 3 microkernel, for example, has only 12,000 lines of code. Microkernels trace their roots back to Danish computer pioneer Per Brinch Hansen and his tenure in Danish computer company Regnecentralen where he led software development efforts for the RC 4000 computer. In 1967, Regnecentralen was installing a RC 4000 prototype in a Polish fertilizer plant in Puławy.
The computer used a small real-time operating system tailored for the needs of the plant. Brinch Hansen and his team became concerned with the lack of generality and reusability of the RC 4000 system, they feared that each installation would require a different operating system so they started to investigate novel and more general ways of creating software for the RC 4000. In 1969, their effort resulted in the completion of the RC 4000 Multiprogramming System, its nucleus provided inter-process communication based on message-passing for up to 23 unprivileged processes, out of which 8 at a time were protected from one another. It further implemented scheduling of time slices of programs executed in parallel and control of program execution at the request of other running programs, initiation of data transfers to or from peripherals. Besides these elementary mechanisms, it had no built-in strategy for program execution and resource allocation; this strategy was to be implemented by a hierarchy of running programs in which parent processes had complete control over child processes and acted as their operating systems.
Following Brinch Hansen's work, microkernels have been developed since the 1970s The term microkernel itself first appeared no than 1981. Microkernels were meant as a response to changes in the computer world, to several challenges adapting existing "mono-kernels" to these new systems. New device drivers, protocol stacks, file systems and other low-level systems were being developed all the time; this code was located in the monolithic kernel, thus required considerable work and careful code management to work on. Microkernels were developed with the idea that all of these services would be implemented as user-space programs, like any other, allowing them to be worked on monolithically and started and stopped like any other program; this would not only allow these services to be more worked on, but separated the kernel code to allow it to be finely tuned without worrying about unintended side effects. Moreover, it would allow new operating systems to be "built up" on a common core, aiding OS research.
Microkernels were a hot topic in the 1980s when the first usable local area networks were being introduced. The same mechanisms that allowed the kernel to be distributed into user space allowed the system to be distributed across network links; the first microkernels, notably Mach, proved to have disappointing performance, but the inherent advantages appeared so great that it was a major line of research into the late 1990s. However, during this time the speed of computers grew in relation to networking systems, the disadvantages in performance came to overwhelm the advantages in development terms. Many attempts were made to adapt the existing systems to have better performance, but the overhead was always considerable and most of these efforts required the user-space programs to be moved back into the kernel. By 2000, most large-scale efforts had ended, although Apple's macOS, released in 2001, uses a hybrid kernel called XNU, which combines a modified OSFMK 7.3 kernel with code from BSD UNIX, this kernel is used in iOS, tvOS, watchOS.
As of 2012, the Mach-based GNU Hurd is functional and included in testing versions of Arch Linux and Debian. Although major work on microkernels had ended, experimenters continued development, it has since been shown that many of the performance problems of earlier designs were not a fundamental limitation of the concept, but instead due to the designer's desire to use single-purpose systems to implement as many of these services as possible. Using a more pragmatic approach to the problem, including assembly code and relying on the processor to enforce concepts supported in software led to a new series of microkernels with improved performance. Microkernels are related to exokernels, they have much in common with hypervisors, but the latter make no claim to minimality and are specialized to supporting virtual machines. Early operating system kernels were rather small because computer memory was limited; as the capability of computers grew, the number of devices the kernel had to control grew.
Throughout the early history of Unix, kernels were small though they contained various device drivers and file system implementations. When address spaces increased from 16 to 32 bits, kernel design was no longer constrained by the hardware architecture, kernels began to grow larger; the Berkeley Software Distribution of Unix began the era of l
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